Update 'project sync' with '--source' to generate a PR from 'source' remote updates.

Nobody has claimed this yet.

## Overview

With the new 'create' setup, it's possible to pickup a 'source' repo. If '--followed', then syncing should update the upstream remote from the source remote. I.e.:

1. `source/master` -> (if it exists)
2. `upstream/master` ->
3. `workspace/master` ->
4. `workspace/` (if it exists)

## Deliverables

* [ ] Add `--source` option to `projects sync`
* [ ] `--source` option is unitary; it cannot be combined with other options.
* [ ] If `--source` option included, check if a `source` remote is available.
* [ ] If no, then `echoerrandexit` with "No 'source' remote found."
* [ ] If yes, then checkout a `source/master` working tree,
* [ ] Pull changes from `source/master` remote,
* [ ] Create work unit branch with description "source sync"
* [ ] Merge `source/master` into newly created workbranch
* [ ] If merge fails, `echoerrandexit` with "Failed to merge updates form 'source/master'. Please complete merge and execute: liq work submit"
* [ ] If merge successful, submit PR.

Start at the `projects sync` command and trace how existing sync options handle remotes, working trees, and work branches. Use `liq work submit` for the final submission flow; done means the `--source` option validates exclusivity, handles the `source/master` merge and failure message, and submits a PR on success.

Written by the indexing model from the issue text.